Colin scriptsit:
If you are talking in terms of material, I was under the impression that to classify as a tartan, the pattern or sett must repeat itself horizontally and vertically, where as a plaid does not.
No, this is not a real distinction. Just look at the modern Welsh tartans that use a different warp and weft. And there are a few old Scottish (unnamed) tartans that have a different warp and weft, as well.

Some people will say that a pattern has to be named to be a tartan and an unnamed pattern is "just a plaid," but this is not a real distinction. Rather like those that say "Mc" is Irish and "Mac" is Scottish and the like -- based on old wives' tales (no offense to old wives on the board!)

Matt